It is true, there is so much variation in shelters and bivy sacks that generalizations always only serve a limited value. My sense is that some might find small shelters, like The One, challenging to inhabit in multi-day rain events because of the relatively small space and the possibility of saturating tent the tent body mentioned above- surrounded on close sides by wet fabric. I don't have experience with this shelter, only heard wonderful things about it, so I'll defer to people who have. It sounds like the hex 3 can be pitched in ways which confer a similar advantage as a tarp.
"Misting" is not something I have experienced, and I understand from other threads on BPL that some controversy exists about the phenomenon- water from outside coming in, condensation from inside being shaken off, etc. From the post above, it sounded like the issue was the tent fabric eventually become somewhat saturated through multiple pitching and stuffing events. I'm assuming that seams were not the issue, though seam sealing is usually an essential measure with single wall tents, as most are aware.
As for the question of relying on a bivy in warm, muggy weather for bug protection at night, I can certainly imagine there is an upper limit of comfort for a bivy in these circumstances which probably varies greatly by the individual and by climate- and this is where, as I understand it, bug nests, nets, etc. come into their own. I am usually in areas where, even in summer, nights cool down, so I can't speak with much experience there.
Prolonged, intense, multi-day heavy rain would probably test the limits of any shelter system in some way, and I've benefited from the many BPL articles that essentially advocate accepting rather than resisting at all costs being wet while hiking. Since I don't own synthetic bags, and am interested in extending the functionality of down as far as possible, I might consider bringing a vb liner with my setup if I anticipated intese, multi-day rain without opportunities for drying out the bag. That way, at a minimum, I had a method at my disposal to dry out the bag- and yes, many would find this a good reason to consider other shelters, and they are certainly right. There are some good recent threads on BPL about hiking in (I believe) the Scottish Highlands, with some interesting arguments for and against UL gear in these conditions, particularly tarp/bivy. Where I spend most of my time, multi-day intense rain is rare, but then again I would never want to not consider hiking in an area simply because of my shelter system, so I'm always eager to learn about this stuff. Although this might create another thread, it would be nice to hear from people who have experienced prolonged rain events and came away with some insight into the merits or limitations of different shelter options!